function typeOf(obj) {
  return Object.prototype.toString.call(obj).slice(8, -1).toLowerCase()
}
console.log(typeOf(''))
console.log(typeOf(0))
console.log(typeOf(true))
console.log(typeOf(null))
console.log(typeOf(undefined))
console.log(typeOf({}))
console.log(typeOf([]))
console.log(typeOf(() => {}))
console.log(typeOf(Symbol('a')))
console.log(typeOf(BigInt(0)))
